In recent years, machining techniques for increasing density and micronization are becoming ever more important in manufacturing steps for semiconductor elements. One such machining technique, CMP (chemical mechanical polishing), has become an essential technique in manufacturing steps for semiconductor elements, for formation of Shallow Trench Isolation (hereunder also referred to as “STI”), flattening of premetal dielectric layers and interlayer dielectric films, and formation of plugs and embedded metal wirings.
Fumed silica-based polishing liquids are commonly used in CMP during conventional manufacturing steps for semiconductor elements, in order to flatten the insulating films such as silicon oxide films that are formed by methods such as CVD (Chemical Vapor Deposition) or spin coating methods. Fumed silica-based polishing liquids are produced by conducting grain growth of abrasive grains by methods such as thermal decomposition with silicon tetrachloride, and adjusting the pH. However, such silica-based polishing liquids have the technical problem of low polishing rate.
Incidentally, STI is used for device isolation on integrated circuits in generation devices starting from design rules of 0.25 μm. In STI formation, CMP is used to remove excess silicon oxide films after formation on substrates. In order to halt polishing in CMP, a stopper film with a slow polishing rate is formed under the silicon oxide film. A silicon nitride film or polysilicon film is used for the stopper film, preferably with a high polishing selective ratio of the silicon oxide film with respect to the stopper film (polishing rate ratio: polishing rate on silicon oxide film/polishing rate on stopper film). A silica-based polishing liquid such as a conventional colloidal silica-based polishing liquid has a low polishing selective ratio of about 3 for the silicon oxide film with respect to the stopper film, and it tends not to have properties that can withstand practical use for STI.
On the other hand, cerium oxide-based polishing liquids comprising cerium oxide particles as abrasive grains are used as polishing liquids for glass surfaces such as photomasks or lenses. Cerium oxide-based polishing liquids have the advantage of faster polishing rate compared to silica-based polishing liquids comprising silica particles as the abrasive grains, or alumina-based polishing liquids comprising alumina particles as the abrasive grains. In recent years, polishing liquids for semiconductors, employing high-purity cerium oxide particles, have come to be used as cerium oxide-based polishing liquids (see Patent document 1, for example).
A variety of properties are required for polishing liquids such as cerium oxide-based polishing liquids. For example, it is required to increase the dispersibility of the abrasive grains such as cerium oxide particles, and to accomplish flat polishing of substrates with irregularities. Using STI as an example, there is a demand for improving polishing selective ratios for inorganic insulating films (such as silicon oxide films) as films to be polished, with respect to the polishing rates for stopper films (such as silicon nitride films or polysilicon films). Additives are often added to polishing liquids to meet this demand. For example, there is known addition of additives to polishing liquids containing cerium oxide-based particles, to control the polishing rates of the polishing liquids and improve the global flatness (see Patent document 2, for example).
Incidentally, as demand increases for achieving greater micronization of wirings in recent manufacturing steps for semiconductor elements, scratches formed during polishing are becoming problematic. Specifically, when polishing using conventional cerium oxide-based polishing liquids, fine scratches have not posed problems so long as the sizes of the scratches are smaller than conventional wiring widths, but they can be problematic when it is attempted to achieve greater micronization of wirings.
A solution to this problem is being sought through studying polishing liquids that employ particles of tetravalent metal element hydroxides (see Patent document 3, for example). Methods for producing particles of tetravalent metal element hydroxides are also being studied (see Patent document 4, for example). Such techniques are aimed at reducing particle-induced scratches, by maintaining the chemical action of the tetravalent metal element hydroxide particles while minimizing their mechanical action.